Signing Hulk Hogan and a few others did move the needle at first. The early days when Hogan first came on were the glory days in terms of ratings on Impact.
You just really need one or two. If anything, it's a good idea to rotate "imported" main eventers in and out to keep your product fresh.
That caveat I have is that it might be worth it to spend moderate money on some crossover stars that might bring eyeballs in from other genres and sports.